The present invention relates to a semiconductor storage device and, more particularly, a semiconductor storage device having a capacitor using a high dielectric constant or ferroelectric film.
A conventional capacitor using a dielectric having a dielectric constant higher than that of a silicon oxide film or a ferroelectric having a dielectric constant higher than that of a silicon nitride film has a large capacitance per unit area. Applications requiring large capacitance with small area, particularly applications such as large-scaled DRAM have been being examined. In such structures, the dielectric having a high dielectric constant is defined as a material having a dielectric constant higher than that of the silicon oxide film. Ferroelectric materials have a spontaneous polarization which can be inverted by an electric field. In particular, as ferroelectrics, complex-metal oxides such as (Ba, Sr)TiO3 (hereinbelow, BST) and Pb(Zr, Ti)O3 (hereinbelow, PZT) are being examined. In order to suppress degradation of the oxides upon film formation, a noble metal such as platinum having oxidation resistance is typically used as a lower electrode. Meanwhile, since an upper electrode is generally formed after film formation, in order to avoid reaction with the dielectric having a high dielectric constant in a heat treatment process after formation of a capacitor, the upper electrode is generally made of platinum.
For example, according to U.S. Pat. No. 5,005,102, a lower electrode has a structure of platinum/titanium nitride/titanium and an upper electrode has a structure of aluminum/titanium/platinum. In particular, with respect to the upper electrode, it is described that aluminum serves as an electrical contact layer, titanium serves as a diffusion barrier layer, and platinum serves as a plate layer.
In the case of fabricating a memory using those elements, after forming a capacitor, a wiring layer for electrically connecting to the capacitor and a wiring layer related to a peripheral circuit part for performing electrical conversion between a memory cell and the outside of the memory chip are formed. In order to obtain electrical insulation between the wiring layers and between the wiring layers and the capacitor, it is necessary to form interlayer insulating films. This process is performed in a reducing or weak acidic atmosphere in order to prevent degradation of the wiring layers. Since a through hole for electrically connecting the peripheral circuit and the wiring layers generally has a shape with a high aspect ratio, which is deep as compared with the size of the opening, tungsten or the like is deposited by a CVD process. The atmosphere at this time is a reducing one. It is known that the capacitor is seriously damaged by being subjected to the treatment in the reducing atmosphere. For example, according to xe2x80x9cMaterial Research Society Symposium Proceedingsxe2x80x9d, Vol. 310, pp. 151 to 156, 1993, it is reported that by forming an SiO2 film by a CVD process, PZT as a ferroelectric loses its the ferroelectricity characteristics and a leakage current increases.
Further, although the characteristics of a semiconductor active device degrade due to a heat treatment in a capacitor fabricating process, a plasma process in a wiring process, and the like, by additionally performing a heat treatment in hydrogen at approximately 400 degrees after completion of the wiring process, the degradation can subsequently be repaired. Consequently, a hydrogen treatment is generally performed after completion of the wiring process. It is known, however, that the hydrogen treatment exerts an influence on the characteristics of the capacitor in a manner similar to the interlayer insulating film process. For instance, according to xe2x80x9c8th International Symposium on Integrated Ferroelectrics, 11cxe2x80x9d, 1996, it is reported that, in the case where SrBi2Ta2O9 (hereinbelow, SBT) is used as a ferroelectric, the capacitor is peeled off or, when the capacitor is not peeled off, a leakage current characteristic largely deteriorates.
The dielectric having a high dielectric constant and the ferroelectric will be generically called a high dielectric constant or ferroelectric film hereinbelow.
It is an object of the invention to obtain a very reliable semiconductor storage device in which the high dielectric constant or ferroelectric film is prevented from degrading.
The object is achieved by providing a capacitor electrode with a film which reduces an amount of hydrogen molecules reaching the capacitor electrode to 1013/cm2 or smaller. It is preferable to use a film by which the hydrogen molecules become 1012/cm2 or smaller.
As a result of examination of the cause of degradation in a treatment using hydrogen, we have found that using platinum as an electrode is related to a degradation process. Specifically, the following mechanism was uncovered. When platinum is used as an electrode, hydrogen molecules are decomposed by the platinum, active hydrogen such as hydrogen atoms and hydrogen radicals are generated, and the active hydrogen is promptly diffused in the platinum, thereby degrading the high dielectric constant or ferroelectric film.
It was also found out that, because of the existence of the mechanism, the capacitor characteristics degrade or an electrode is peeled off even at a low temperature such as 300xc2x0 C. At such a low temperature, it has not previously been expected that the high dielectric constant or ferroelectric is reduced and degraded.
By providing a film which prevents hydrogen molecules from reaching the electrode as much as possible, the high dielectric constant or ferroelectric film can be prevented from degrading.
To be specific, it is sufficient to provide a film whose adsorption of the hydrogen molecules is 1013/cm2 or smaller, preferably, 1012/cm2or smaller. By providing such a film which substantially does not adsorb hydrogen (hereinbelow, called an xe2x80x9cadsorption inhibiting layerxe2x80x9d), the amount of hydrogen molecules reaching a platinum film as a part of the capacitor electrode is decreased and, as a result, the amount of active hydrogen can be reduced. Thus, degradation and peeling in the wiring forming process of the high dielectric constant or ferroelectric capacitor are suppressed and improvement in the long-term reliability is realized. As a material of the film, silver, aluminum, silicon, lead, bismuth, gold, zinc, cadmium, indium, germanium, and tin are effective. Since the surface of each of these materials has an atomic arrangement which prevents the adsorption of hydrogen, it is effective in preventing the adsorption. As described above, by providing a layer which does not adsorb hydrogen much as compared with platinum, an effect on suppression of generation of the active hydrogen is produced. In the case where only an aluminum film is used, since the adsorption of hydrogen is relatively good, it is preferable to further provide a diffusion preventing layer which will be described hereinbelow. When the films are formed so as to be in contact with the platinum electrode, mutual diffusion occurs. It is therefore preferable to provide a reaction preventing layer such as a titanium nitride film or a tungsten nitride film therebetween.
A film in which the diffusion of hydrogen molecules is 1013/cm2 or smaller, preferably, 1012/cm2 or smaller may be provided. By providing a film which substantially prevents the diffusion of the hydrogen molecules (hereinbelow, called a xe2x80x9chydrogen diffusion preventing layerxe2x80x9d), the diffusion amount of the hydrogen molecules becomes extremely small, the amount of hydrogen molecules reaching the capacitor electrode is decreased, and the amount of active hydrogen generated by the action of the capacitor electrode can be reduced. As a hydrogen diffusion preventing layer, specifically, besides tungsten, a conductive oxide such as ruthenium oxide, iridium oxide, palladium oxide, osmium oxide, or platinum oxide, ruthenium, iridium, palladium, osmium, or an oxide of an alloy of any of these materials can be mentioned. In the case where the capacitor electrode is made of any of these materials without providing the adsorption inhibiting layer, since the reaction between any of the materials and the platinum electrode does not occur to a large extent, it is unnecessary to provide a reaction preventing layer.
Further, when the capacitor electrode is provided with a stack of the adsorption inhibiting layer and the hydrogen diffusion preventing layer, the amount of hydrogen molecules reaching the capacitor electrode is further reduced, so that it is more effective. In this case, it is sufficient that an amount of hydrogen of 1012/cm2 or less reaches the electrode through both of the adsorption inhibiting layer and the hydrogen diffusion preventing layer. In the case where the layers are stacked and one of the adsorption inhibiting layer and the hydrogen diffusion preventing layer is made of an oxide, in order to prevent the reaction between the layers, it is preferable to provide a reaction preventing layer between them. When both of the layers are made of oxides, it is unnecessary to provide the reaction preventing layer.
The reaction preventing layer may be made of titanium, a titanium alloy, or a titanium nitride. Besides them, any of tungsten, tantalum, molybdenum, nitrides of these materials, and the like can be used as a hydrogen diffusion preventing layer and reaction preventing layer.
When the total thickness of the capacitor electrode, the hydrogen diffusion preventing layer, and the adsorption inhibiting layer is 20 nm or more, a beneficial effect to a certain extent can be expected. When it exceeds 0.5 xcexcm, the thickness becomes difficult to form the structure. Consequently, it is preferable that the film thickness lies in a range from 20 nm to 0.5 xcexcm.
The material of the capacitor electrode is not limited to platinum but may be ruthenium, iridium, palladium, nickel, osmium, rhenium, or a material whose main component is a conductive material of an oxide of any of these materials.
Although sufficient effects can be obtained when the hydrogen diffusion preventing layer and the adsorption inhibiting layer are formed in the upper part of the capacitor electrode, it is more effective when the layers are formed not only in the upper part but also on the sides. Further, when they are formed under the capacitor electrode, it is effective in preventing invasion of the hydrogen molecules diffused from the wafer substrate side into the capacitor electrode. A specific description will be given hereinbelow.
A capacitor and a semiconductor active device are provided and the hydrogen diffusion preventing layer is interposed between the capacitor and the semiconductor active device. It is more preferable that a hydrogen adsorption preventing layer is disposed on the capacitor. In the capacitor, two electrodes may be arranged vertically or horizontally. When the two electrodes are arranged vertically, the hydrogen adsorption inhibiting layer may construct a part of the upper one of the two electrodes.
It is preferable that a part of the hydrogen diffusion preventing layer constructs a part of a connection plug which electrically connects one of the two electrodes to the semiconductor active device.
Further, it is preferable that a part of the hydrogen diffusion preventing layer constructs a part of an interlayer insulating film interposed between the capacitor and the semiconductor active device. In this case, preferably, the hydrogen diffusion preventing layer is an oxide insulator. As such a material, for example, a material having the main component of an aluminum oxide or a cerium oxide can be used. The aluminum oxide or cerium oxide can be used by being contained in SiO2 typically used for the insulating layer. When the aluminum oxide or the cerium oxide has 5 weight % or more, a certain extent of the desired effect is produced. When it is 10 weight % or more, more of the desired effect is realized. The upper limit of the value of the aluminum oxide is regulated from the viewpoint of the workability. The upper limit of the value of the cerium oxide is regulated from the viewpoint of the insulation performance.
Further, according to the semiconductor device of the invention, it is preferable that a second semiconductor active device is provided in an area which is different from the area where the capacitor is disposed and the hydrogen diffusion preventing layer is not formed on the second semiconductor active device.
As a result of the above-mentioned features, defective insulation and peeling of the electrode caused by a treatment in a reducing atmosphere, such as metal film CVD, and a treatment using hydrogen, such as an interlayer insulating film forming process, can be prevented and the long-term reliability can be improved.